Prior art control circuits of ultrasonic disintegrators operate at a constant operating frequency which is matched with the mechanical oscillating system of the ultrasonic transducer and which is operable only in a narrow frequency range. An ultrasonic disintegrator is composed of a h.f. generator converting the electrical mains power into h.f. power and of a sound transducer generating, in conjunction with an amplitude amplifier adapted as a .lambda./2 oscillator and a sonotrode, mechanical longitudinal oscillations of high power with large amplitudes.
In contrast to ultrasonic cleaning devices, ultrasonic disintegrators can be employed, in particular in the laboratory equipment technology, also for crushing or shattering smallest solid components in liquid substances, in order, e.g., to homogenize and create finest emulsions of difficult to mix substances.
In the German patent office publication DE 32 22 425 A1, a generator for driving a piezo-resonator is described. A safe oscillation build-up of the generator is to be secured even when the oscillation frequency of the resonator is reduced. For the purpose of a safe oscillation build-up, the frequency of the signals fed to the sound transducer is periodically modified about the resonance frequency of the transducer, until the feedback amplitude exceeds a certain limit. It is disadvantageous, among other reasons, that the circuit has to be matched to the resonance frequency of the ultrasonic oscillator, so that the operation of the ultrasonic oscillator cannot follow the modifications of the parameters of the ultrasonic oscillator.
In the EP 0 340 470 A1, a circuit for the excitation of an ultrasonic oscillator is described, which follows-up the excitation frequency according to the modifications of parameters of the ultrasonic oscillator. For this purpose, a measured quantity corresponding to tile attenuation of the ultrasonic oscillator is formed and compared to a predetermined maximum allowable attenuation. If the attenuation of the ultrasonic oscillator is smaller than the maximum allowable attenuation, the control voltage is also regulated, depending on the measured quantity.
Further, from DE-Z "radio mentor", 4/1965, p. 280-281, an ultrasonic welding device is known in tile art, including a generator provided with an automatic frequency control. For this purpose, a voltage is derived from the oscillator over a piezo-electrical trunk, the voltage being proportional to the oscillator amplitude. The power transmitted to tile material to be welded can thus be held constant during the welding time.
The narrow frequency range wherein ultrasonic transducers are operable, leads to different sonotrode types having substantial geometric differences which can only be difficultly operated with a single generator, that worn sonotrodes have to be replaced prematurely, and that a high production accuracy is required for the sonotrodes.
The disadvantages of the state of the art are also caused by large variations of the mechanical resonance frequency, as they can be caused by production tolerances, cavitation wear of the sonotrodes, thermal length variation of the sonotrodes or assembly mistakes. These variations in frequency may cause the transducers to not build-up oscillations and/or overload or destroy the power end stage of the control of the ultrasonic transducers.